Spinal surgery often requires removal of the existing intervertebral disc tissue located between adjacent vertebrae and replacement thereof with an intervertebral implant which may take the form of a cage or other fusion device or an artificial disc which may be of the type which allows movement of the adjacent vertebrae with respect to each other.
In any event, it is necessary to initially separate the adjacent vertebrae from each other and to retain them apart during the surgical procedure to clear out the existing disc tissue, perform a spinal decompression, prepare the bony endplates to receive the implant, and insert the intervertebral implant.
It has been known heretofore to distract adjacent vertebrae away from each other using an instrument having a pair of anchor pins, one pin anchored in each of the adjacent vertebrae, wherein one anchor pin is received in a tube that is fixedly attached to a crossbar and the other pin is received in a tube that is movably attached to the crossbar. Thus, by moving the movable tube away from the fixed tube, adjacent vertebrae are forced away from each other or distracted. Typically, the crossbar is located some distance away from the vertebrae being distracted. This results in a moment arm at the point where the tubes are coupled to the crossbar. Because the anchor pins are solidly embedded in the vertebrae, the moment arm causes the upper portion of the tubes to move farther apart from each other than the lower portion of the tubes. The resultant force operates to slide the tubes upwardly on the pins away from the vertebrae until one or both of the anchor pins are no longer fully engaged within the tubes. Manipulation and impaction of instruments during the surgical procedure can also exacerbate this relative movement of the pins within the tubes.
A need exists for a mechanism which restricts the movement of the distractor tubes away from the vertebrae when the distractor is in use. A further need exists for a distractor which restricts movement of the distractor tubes away from the vertebrae which is easily manipulated during a surgical procedure. It would be advantageous if the distractor could be coupled to the anchor pins using a mechanism that is mounted on the distractor. It would be beneficial if this mechanism were automatic and did not require active engagement by the user, thus simplifying surgical use. Moreover, it would be beneficial if the anchor pins could be coupled to the distractor without the need for the anchor pins to extend above the distractor tubes.